This invention relates to electroluminescent devices and in particular to electroluminescent devices based on organic light emitting diodes (OLEDS).
Known organic light-emitting diodes (OLEDs) generally comprise an emissive electroluminescent organic semiconductor material layer formed between a cathode and an anode formed on a glass substrate, whereby the electroluminescent organic semiconductor material emits light when a voltage is applied across the anode and cathode electrodes.
In order for visible light to be emitted from the OLED, at least one of the electrodes must be transparent to radiation in the 350 nm-800 nm wavelength range.
OLEDs are considered to be suitable candidates for the next generation of displays, e.g., flat panel displays because of their advantages over conventional technologies used in traditional displays, e.g., liquid Crystal displays (LCDs) and plasma display panels (PDPs). OLEDs are also being increasingly used in lighting applications, replacing the more common incandescent bulbs.
Such advantages over LCDs and PDPs include, lower fabrication costs, light-weight, flexible plastic substrates, wider viewing angles & improved brightness, increased power efficiency and faster response time. However, whilst efficient in comparison to LCDs and PDPs, the efficiency of the OLED is limited, due to incomplete light extraction from the active, light-emitting layer due to losses in the OLED.
FIG. 1 shows in section, a conventional OLED 1 of the prior art, whereby an organic electroluminescent material layer 2 is located between two electrodes; a transparent anode 4 and a reflecting cathode 6.
The specific type of electroluminescent layer to be used may vary depending on the application of the OLED. For example, such material for the electroluminescent material may include organometallic chelates, for example Alq3, fluorescent and phosphorescent dyes and conjugated dendrimers. Alternatively, organic polymer molecules may be used, whereby typical polymers include derivatives of poly (p-phenylene vinylene) and polyfluorene. Substitution of side chains onto the polymer may determine the colour of emitted light or the stability and solubility of the polymer for performance and ease of processing. Furthermore, a polymer such as poly (n-vinylcarbazole) may be used as a host material to which an organometallic complex is added as a dopant. Iridium complexes such as Ir(mppy)3 or complexes based on other heavy metals such as platinum may also be used.
The anode 4 is fabricated from a transparent material (e.g., indium-tin-oxide (ITO)), whilst the cathode 6 may be fabricated from a reflective metal (e.g., magnesium-silver or lithium-aluminium alloy). A protective substrate (not shown) is then deposited over the anode 4. The substrate may be flexible. Alternatively, the anode, cathode and electroluminescent material may be deposited on the substrate.
In operation, when a voltage 10 is applied across the electrodes 4 and 6, holes from the anode 4 and electrons from the cathode 6 are injected into the organic layer 2. These holes and electrons migrate through the organic layer 2 until they meet and recombine to form an exciton. Relaxation from the excited to ground states then occurs, causing emission of light 12 through the transparent anode 4.
Furthermore, it is known to use different layers and materials to increase the efficiency of OLEDs, whereby hole and electron injection and/or blocking and/or transport layers are used to optimise the electric properties of the OLED. For example, hole injection (HIL), e.g., Cu/Pc and/or hole transport layers (HTL), e.g., aNPD, Triarylamines, and/or electron transport (ETL), e.g., Alq3 and/or Hole Blocking layers (HBL), e.g., BCP may be used to improve electrical efficiency as required. When a voltage 10 is applied between the anode and the cathode in the OLED having the above-described structure, holes generated in the anode move to the emission layer through the HIL and the HTL, and electrons generated in the cathode move to the emission layer through the HBL and the ETL. The holes and electrons moved to the emission layer are recombined in the emitting layer to emit the light. The light generated in the emission layer is emitted to the outside through the anode. Whilst the HIL, HTL, ETL, and HBL address the electrical efficiency of OLEDs, one of the key challenges in the design of OLEDs is to optimize their light extraction efficiency.
For conventional OLEDs as depicted in FIG. 1, light extraction inefficiencies exist because light generated within a high-index organic material has difficulty propagating into the surrounding lower-index Anode/Glass substrate owing to total internal refection (TIR) at the glass/air interface, coupling to dielectric waveguide modes of the organic layers, in-plane emission, and dissipation into the metal contacts of the OLED. Approximately, 30% of the generated photons remain trapped in the glass substrate and 50% in the organic layers. Therefore, output coupling efficiency of known OLEDs is approximately only 20%.
Various approaches for improving the optical out coupling efficiency of OLEDs have been put forward. For example, the planar substrate/air interface may be modified in order to reduce repeated TIR, e.g., by using a micro lens array, or a large half sphere lens on the substrate surface.
Other methods of out-coupling efficiency attempt to extract the light trapped in the organic/ITO layers for example by using a low refractive index porous aerogel, micro-cavity effects, or an embedded low-index grid photonic crystal pattern on the glass substrate. However, using these methods, the improvement in relation to OLED efficiency is limited. Moreover, some of these methods have disadvantages associated with them such as a reduction in electrical efficiency, a decrease in lifetime, a viewing angle dependent colour, complicated fabrication processes and high costs. For instance, eliminating TIR using antireflective coatings does not work as each of the layers will refract the light and ultimately the final interface with air will still meet the TIR condition. In addition, establishing gratings at pitches less than half the UV exposure wavelength is very difficult.